1. Field of the Invention
The present invention relates to semiconductor devices and, in particular, to a semiconductor device having a nitride semiconductor layer.
2. Description of the Related Art
A nitride semiconductor layer is usually formed on an inexpensive silicon substrate or sapphire substrate. However, the lattice constants of these substrates greatly differ from the lattice constant of the nitride semiconductor layer and they also have different coefficients of thermal expansion. Therefore, a high amount of distortion energy is generated in the nitride semiconductor layer formed on the substrate by epitaxial growth. As a result, a crack easily appears in the nitride semiconductor layer and crystal quality is easily reduced.
To solve the above-described problem, a method of disposing a buffer layer formed of stacked nitride semiconductor layers between a silicon substrate and an active layer formed of a nitride semiconductor, is proposed (refer to, for example, Patent Document 1).
A semiconductor wafer having a buffer layer of Patent Document 1 is depicted in FIG. 4.
In FIG. 4, a buffer layer 3 is provided between a silicon substrate 2 and an active layer 4, and the buffer layer 3 has a first multi-layer structure buffer region 5, a second single-layer structure buffer region 8 which is formed of GaN and is provided on the first multi-layer structure buffer region 5, and a second multi-layer structure buffer region 5′ provided on the second single-layer structure buffer region 8.
Furthermore, the first multi-layer structure buffer region 5 and the second multi-layer structure buffer region 5′ each have a multi-layer structure in which a sub multi-layer structure buffer region 6 and a first single-layer structure buffer region 7 which is formed of GaN and is thinner than the second single-layer structure buffer region 8 are repeatedly stacked.
Moreover, the sub multi-layer structure buffer region 6 has a multi-layer structure in which a first layer formed of AlN and a second layer formed of GaN are repeatedly stacked.
In Patent Document 1, it is disclosed that a warpage in a semiconductor wafer is reduced by forming the first layer with a nitride semiconductor containing aluminum in a first proportion and making the proportion (including zero) of aluminum in the second layer, the first single-layer structure buffer region 7, and the second single-layer structure buffer region 8 smaller than the first proportion.